A weight of a vehicle, which may take the form of a vehicle train, is an important parameter in many functions in its control system. A vehicle is considerably affected by its weight in many situations, so it is very important to be able to estimate it correctly. Its weight typically forms part of models of the vehicle which are used for sundry calculations and forms of control in the vehicle.
The weight of a vehicle capable of carrying heavy loads, e.g. a bus capable of carrying a large number of people or a truck capable of carrying various types of heavy loads, may vary considerably. For example, a truck weighs considerably less when unladen than when fully laden. A bus likewise weighs considerably less when empty than when full of passengers. Variations in the weight of a car are considerably smaller than in the case of vehicles intended to carry heavy loads, but the difference between a car empty and fully laden, especially when a fully laden car may also have a laden trailer coupled to it, may be large relative to the low weight of the car itself.
The weight of a vehicle affects its running resistance and is therefore an important parameter in, for example, automatic gear choice. Automatic gear choice takes place, for example, in an automatically operated manual gearbox, for which it is important to be able to determine an actual running resistance and hence which gear to choose in a given situation.
Now the topography of a section of road affects a vehicle also depends greatly on the vehicle's weight, which is a decisive factor in how much it respectively accelerates and decelerates downhill and uphill. Vehicle weight is therefore also an important parameter in cruise controls which take the topography of a road section into account, so-called look-ahead cruise controls, in which the amount of engine torque employed in a situation depends on how the topography of road sections ahead will affect the vehicle's speed. The vehicle's weight is of course also an important parameter in conventional cruise controls.
Various methods are currently applied to estimate a vehicle's weight. One such method uses information from an air suspension system of the vehicle. The air suspension system measures axle pressures on all of the axles which have air suspension and conveys this information to a control unit as a basis for it to calculate the vehicle's weight. This method works well if there is air suspension on all of the axles, but does not work satisfactorily, or at all, if one or more axles have no air suspension. This method is for example particularly problematical in vehicle trains comprising trailers or semitrailers which do not report axle loads, which may be relatively common in cases where more or less unknown trailers are often coupled to a vehicle train during use of the vehicle. This method is also problematical during operation of the vehicle, since the axle pressure will vary when the vehicle travels over uneven portions of the carriageway, potentially leading to incorrect weight estimation due to unsteady axle pressures.
Other known weight estimation methods are based on acceleration, using the fact that it is possible to calculate the vehicle's weight from a force which the engine imparts to the vehicle and an acceleration resulting from this force. The force from the engine is known in the vehicle but such methods involve having to measure or estimate the acceleration.
According to a method, the acceleration is estimated by making a derivation of the vehicle's speed. This method works well at high accelerations, i.e. in low gears at relatively low vehicle speeds, but a disadvantage is that this method is affected by road gradient, necessitating the derivation, since the road gradient is a parameter unknown to the system.
Another method estimates the acceleration by means of an accelerometer. The accelerometer-based method has an advantage in that acceleration is measured directly, but only a limited, number of today's vehicles have an accelerometer, which means that this method is not generally applicable on all vehicles. The current accelerometer-based method also suffers in that the accelerometer signal is noisy, reducing the accuracy of the method.
Another method estimates the acceleration during gear changes. This method is based on the assumption that the running resistance does not change during a gear change and on therefore comparing the vehicle's acceleration before, during and after gear changes in order to determine its weight. This method results in very unsatisfactory estimates of vehicle weight.
Acceleration-based weight estimates generally have disadvantages in that certain running requirements have to be fulfilled for good estimation to be possible. It is not always certain that these requirements are fulfilled during a run, in which case good weight estimation is not possible. For example, acceleration-based weight estimation requires full acceleration in low gears to achieve a reliable result. As such full acceleration does not always occur during a run, if for example the vehicle begins its run downhill, e.g. on an entry slip from a filling station to a motorway, where the downgrade makes it possible to accelerate relatively gently before maintaining a substantially constant speed for the rest of the journey, these methods often do not provide a good estimate of vehicle weight.
Thus the previous known weight estimation methods are not always applicable and/or fail to provide reliable estimates on every kind of run.